It has long been recognized that optical image correlators may have useful applications for pattern recognition. One class of correlators are known as "joint Fourier transform optical correlators." In these devices, conveniently described with reference to FIG. 1, Fourier-transform lens 80 operates on a pair of coherent images representing a reference R and an unknown object S. The resulting optical intensity distribution in the focal plane of the Fourier-transform lens is recorded in photorefractive medium 25. The output of the correlator is generated by a Fourier-transform lens (also shown in the figure as lens 80) operating on the recorded pattern. Each of two side regions of the output image (symmetrically displaced from the center by the separation between R and S) contains an intensity distribution corresponding to the cross correlation between R and S. The position of a correlation peak identifies the location of a feature of R that resembles S. The height of the peak measures the degree of similarity. A correlator of this kind is described, e.g., in H. Rajbenbach et at., "Compact photorefractive correlator for robotic applications," App. Opt. 31 (1992) 5666-5674. This system used a crystal of Bi.sub.12 SiO.sub.20 (BSO) as the photorefractive medium. With this material, a typical response time of about 50 ms was achieved. Using a crystal about 1 mm thick, diffraction efficiencies of 0.1% -1% were obtained.
A second class of correlators are known as "Vanderlugt optical correlators" . These devices are described, e.g., in D.T.H. Liu et al., "Real-time Vanderlugt optical correlator that uses photorefractive GaAs," Appl. Optics 31 (1992) 5675-5680. In these correlators, conveniently described with reference to FIG. 2, the Fourier transform of, e.g., the S image is written in photorefractive medium 25 by interfering it with reference beam 5, which is typically a plane wave. The output of the correlator is generated by using lens 84 to create a Fourier transform of the R image, which is impinged on the photorefractive medium. As depicted in the figure, lens 82 is used both to generate the Fourier transform of the S image, and to generate the inverse Fourier transform of the output from the photorefractive medium.
The system described by D.T.H. Liu et al. used a crystal of gallium arsenide, 5 mm thick, as the photorefractive medium. Diffraction efficiencies less than 0.1% were obtained. The shortest response time measured was 0.8 ms at a laser intensity of about 1.5 W/cm.sup.2.
There remains a need for photorefractive media that are more sensitive and that respond more quickly to low-power beams. That is, optical processing has hitherto been limited to video rates or the like. Substantially greater processing rates are desirable for, e.g., applications in which great volumes of image data need to be processed. Moreover, the density of resolvable spots in the input images R and S is limited by the thickness of the photorefractive medium. Greater sensitivity is required in order to achieve diffraction efficiencies of 1% or more in thicknesses substantially less than 1 mm.